post 11

Earth’s Internal Structure

What layers make up the Earth? How does composition change with depth?

How do physical properties change?

 How do we know?  Crustal structure?

 Deeper structure: e.g., that the outer core is liquid?

[Text: 9.1]

Earle, S. (2016): Online text Fig. 9.6b

[last class]

How do we know that the outer core is liquid? (1) S waves cannot travel through it (2) P waves are slowed

Earle, S. (2016): Online text Fig. 9.6a

How do we know that the outer core is liquid? (3) Slowed P waves must be refracted downward (not upward)

 shadow zone (where no P waves arrive)

diagram: refraction at outer core

Earle, S. (2019): Online text Fig. 9.1.6

Other seismic discontinuities

In mantle: 410 km, and 660 km: - phase changes to denser

mineral structures

Deep: 2900 km – core/mantle

boundary (CMB) 5150 km – inner core/outer

core boundary

Earle, S. (2016): Online text Fig. 9.6a

7

Aegean S. Kuril Izu-Bonin

Kearey et al. (2009): Global Tectonics, 3rd edn., Wiley-Blackwell, Plate 9.2

How do we know that the inner core is solid iron?

 Gravitational pull exerted by Earth  requires high-density iron inner core

 The whole Earth oscillates after large earthquakes  inner core must be solid

Earth’s Internal Structure - Summary

 3 main sections by composition: crust, mantle, and core

 Physical properties change with depth in response to increased P and T

 5 main sections by physical properties: rigid lithosphere, partially molten asthenosphere, rigid mesophere, liquid outer core, solid inner core

 Boundaries in the Earth’s interior act as seismic discontinuities – abrupt changes in the velocity of seismic waves travelling through the Earth

Stratigraphy and Geologic Time

 Why is it important to document Earth history?  How do we know that one rock is older than another

(relative age)?  Principles..  Fossil record..

 How do we know the age of the Earth, and how has our understanding changed over time?

 How can we use radioactivity to determine the (absolute) age of a rock?

 How was the Geologic Timescale put together?

[Text: 8.1-8.6]

Why document Earth history?

 The past may be the key to the future: changes occur in cycles, patterns repeat

 How has the Earth changed? - e.g. CO2 levels, climate, sea level, landmasses, biological evolution & extinctions, Wilson cycles

HOWEVER:  Almost all of Earth’s history predates humans

How is Earth history documented?  The Rock Record

Sediments laid down in layers (strata)  a stratigraphy of events: a record of earth processes over geological time

deep

shallow

deep

shallow

Image: http://explanet.info/Chapter02.htm

Reading the book (the stratigraphic record) Main concepts:

1) Beds (strata, sedimentary units) – horizontal – bound by bedding planes

2) Multiple beds make up a stratigraphic ‘sequence’ - bound by erosional episodes due to fluctuations in sea-level, uplift

3) Strata contain fossilized flora and fauna 4) The rock record is incomplete at any one place (gaps in time)

L. Leonard

Biostratigraphy

Fossils  Used in correlation of strata

 Occur over a stratigraphic range

 Organisms: also provide info on environment – some organisms tolerate very wide environmental

conditions; others do not E.g., different trilobite species: bottom dwellers vs. floaters vs. swimmers

Image: http://www.wilsonmuseum.org/treasures /treasures_new2.html

Two ways to view geologic time:

(1) Relative time - dating by sequence of events

- "older" vs. "younger" - placed in order

(2) Absolute time - numerical dating using radioactive

decay in minerals

Geologic Time

Image: http://www.evolution.berkeley.edu/evosite/lines/IIIAchronology.shtml

How to establish the order of geological events?

Relative age dating: Principles 1) Superposition 2) Original horizontality 3) Faunal (& floral) succession 4) Inclusions 5) Cross-cutting relations

(1) Principle of Superposition: In an undisturbed stratigraphic sequence, the rocks at the top

are youngest

Colorado River, Utah Image: Lutgens, Tarbuck, Tasa (2006): Essentials of Geology, 9th edn., Pearson.

[17th century: Nicolas Steno]

(2) Original Horizontality: Sedimentary layers are deposited in horizontal units/beds

Inclined layering  layers were tilted from initial horizontal orientation some time after deposition

Images: Lutgens, Tarbuck, Tasa (2006): Essentials of Geology, 9th edn., Pearson.

Possible Problem: overturned strata in mountain belts (deformation)

 need to know ‘which way is up’ in a sedimentary package

Image: https://www.nps.gov/parkhistory/online_books/geology/publications/ pp/296/sec2a-2.htm

http://college.cengage.com/geology/resources/protected/physicallab/thelab/geolog icmaps/activities/activity1/activity1.htm

Large gaps of time between deposition of layers (strata): unconformities

“Hutton’s unconformity”, Siccar Point, Scotland

How does an angular unconformity form?

65 Myr missing

deposition,

tilting,

erosion,

deposition

Image: Hamblin & Christiansen (2003): Earth’s Dynamic Systems, 10th edn., Prentice Hall

time

Angular unconformity Beds above: ~ horizontal Beds below: dip down to right (~500 Myr history missing)

Earle, S. (2016): Online text Fig. 8.8

(3) Faunal (& Floral) Succession: Sedimentary layers contain fossilized flora & fauna

Organisms succeed each other vertically in a specific, reliable order  fossil record  Rocks with similar fossils are (generally) of similar age

e.g., Neanderthal bone (young) never found in same strata as a Tyrannosaurus Rex (much older)

Earle, S. (2016): Online text Fig. 8.10

66 Ma252 Ma541 Ma

Tarbuck, Lutgens, Tsujita (2015): Earth: Introduction to Physical Geology, 4th Cdn. Edn., Pearson.

Which are useful index fossils?

(4) Inclusions Older rocks “included” in younger rocks – e.g., blocks eroded from country rock by intruding magma

Earle, S. (2016): Online text Fig. 8.6a

xenolith

Earle, S. (2016): Online text Fig. 8.6b

Sedimentary inclusion: “rip-up” clast

(5) Cross-cutting relations Older rocks are “cross-cut” by younger rocks or features

(e.g., dykes, faults, erosion surfaces)

http://www.geosociety.org/Earthcache/Images/block%20diagram1182008.jpg

How many? Relative age?

Image: Hamblin & Christiansen (2003): Earth’s Dynamic Systems, 10th edn., Prentice Hall

What principles explain the sequence of events A to T?

http://www.wiringdiy.com/static/block-diagram-of-well-who-knows-where-just-try-to-put-things-in-1541910.jpeg

Layers B to G are younger than A: principle of superposition H younger than A-G: _____________________ I younger than A-H: _____________________ J,K,L younger than I: _____________________ M younger than A-L: _____________________ N,O younger than M: _____________________ P,Q younger than A-O: _____________________ R younger than A-Q: _____________________ S,T younger than A-Q: _____________________

How old is the Earth?

Archbishop James Ussher (1600’s):

early biblical view: Earth age #1: 6 days + 6000 yrs

 so much in so little time!

Catastrophism: Earth history must be shaped by sudden, violent processes (e.g., biblical flood)

Image: Wikimedia Commons

Uniformitarianism Sir James Hutton (late 1700’s):

Processes forming sediment layers today are gradual

 Uniformitarianism: “the present is the key to the past”

 Earth age #2: very old (at least millions of years)

“no vestige of a beginning, no prospect of an end”

– radical idea at the time – same conclusion later reached by Lyell, Darwin

http://www.smithsonianmag.com/history/fa ther-modern-geology-youve-never-heard- 180960203/?no-ist